1. Field of the invention
The present invention concerns a channel selector device for multiple access direct transmission systems between mobile stations.
2. Description of the prior art
By direct transmission between mobile stations is meant a non-relayed mode of communication, i.e. one with no infrastructure, between two or more mobile stations of a common group in a given geographical area. Given the limited power of the mobile stations, the range of such communications does not exceed a few kilometers. This mode of operation is particularly beneficial as a complement to the usual relayed mode offered by a mobile radio system. It enables communication even outside the coverage area of the infrastructure. However, it may also be used within the coverage area of the infrastructure, for example in order to avoid loading the system or because only a part of the group is in the coverage area.
By multiple access system is meant a system of sharing transmission resources enabling a plurality of simultaneous calls on separate channels using one of the following three forms of multiplexing: frequency-division multiplexing (FDMA), time-division multiplexing (TDMA), and code-division multiplexing (CDMA).
In relayed mode the allocation of radio resources is usually coordinated by the infrastructure using a predefined cellular plan and dynamic allocation of the various channels offered by the multiple access system in question to the various calls. In direct mode the absence of any such centralized coordination requires channel selection rules (usually manual) and a certain degree of discipline by users. In practice the users of a group agree beforehand to use a given channel (this is typical in professional mobile radio: PMR) or to use a xe2x80x9crallyingxe2x80x9d channel (or control channel) common to more than one group before xe2x80x9cswitchingxe2x80x9d to another dedicated channel (digital short range radio: DSRR).
Specific problems arise when the direct mode is designed to complement an existing TDMA (time-division multiple access) relay mode: this is the case with the GSM and TETRA systems, for example. The natural solution would be to reproduce the same TDMA operation in the direct mode in order to minimize the specification effort and to facilitate the implementation of a mobile terminal integrating both the relayed mode and the direct mode. In the direct mode, however, in the absence of any absolute time reference outside the coverage area of the infrastructure, it does not seem obvious to manage the use of the channels by time sharing as in TDMA. Assume, for example, that two groups start calls in direct mode on the same frequency but on different channels (i.e. in different time slots); as on the face of it the two groups can have any time reference, and their time references are independent, it can very well happen that the time slots used overlap at least in part with the result that the calls interfere with each other, as shown in FIG. 1 for the case of two calls COM1 and COM2 respectively using time slots 4 and 3 of a frame structure with eight time slots numbered 1 through 8, for example.
In the TETRA system this problem has been solved in a simple way by assuming that only one call per frequency is possible in direct mode. This is a simplifying solution but is greedy in terms of spectrum use. From this point of view, what may be acceptable in the case of the TETRA system in which frequencies are spaced by 25 kHz becomes exorbitant in the case of the GSM system where the spacing is 200 kHz.
The uncoordinated nature of the calls in direct mode is therefore particularly penalizing in the case of a TDMA system, where it seems on the face of it to be difficult to envisage more than one call per frequency in a given area without risk of calls interfering with each other. Even in the case of direct mode FDMA or CDMA transmission, however, malfunctions in connection with calls in the same area can happen frequently, for two main reasons:
the traffic channel selected at the start of the call may in fact be busied by a call in the same area that was inactive at the time of the measurement but becomes active thereafter;
two calls in two separate but closely spaced areas may be set up on the same channel and may thereafter interfere with each other if the two groups move closer together.
A direct mode with FDMA digital transmission in 25 kHz channels is defined in the digital short range radio (DSRR) system. There is also a repeater mode in which one of the mobile stations serves as a relay. The frequency band (in the 900 MHz range) comprises two control channels and 76 traffic channels. The control channel is used in the call set-up phase to transmit synchronization and signaling information and the traffic channel is selected either by the calling mobile station or by the relayed mobile station in repeater mode. In all cases, the selection of the traffic channel is based on first looking for a free channel using appropriate radio measurements. This selection mode guarantees correct operation only at the start of the call, however. The channel selected may in fact have been busied by another call that was not active at the time of the measurement but which becomes active afterwards. Moreover, given the mobile nature of the users, two groups initially located in different areas may select the same traffic channel and then interfere with each other upon moving close together.
One object of the present invention is to avoid the aforementioned problems.
The present invention consists in a channel selector device for a multiple access system of direct transmission between mobile stations, including means for carrying out measurements on various channels during calls in order to determine if there is a better channel than the traffic channel used for said call, said better channel constituting the new traffic channel to be used.
Other objects and features of the present invention will emerge from a reading of the following description of one embodiment of the invention given with reference to the appended drawings.